edge_index doesn't say if the edge has to be canonical or not.

[adamnovak]

VectorizableHandleGraph::edge_index should state that the edge_t has to be from for_each_edgeor edge_handle (i.e. in canonical order and orientation).

[jeizenga]

Seems to me it should be pretty easy to canonicalize internally

[adamnovak]

It kind of depends on if we want people to construct their own edge_t values ever, or if they are supposed to be opaque like handles.

Since there's no accessors into them other than .first and .second, I guess we should actually regard them as non-opaque and canonicalize the input edges.